Unless you’re also replacing the trim, set it to the side so you can put it back on after you install your new door.

Each door can weigh 100 pounds (45 kg), so get someone to help you remove the old door and set it aside.

Get rid of any boards or shims that might’ve been added to make the old door fit as well. You should have nothing but the studs on the sides and the header on the top.

Add shims where needed to correct so all four sides of the rough opening are plumb and square before you measure for your door.

If your rough opening is square, level, and plumb, you should get the same measurements no matter where on the rough opening you measure.

When you get the door home, go ahead and take it around to where you’re going to install it and leave it outside—you’ll be installing it from the outside, so there’s no need to try to get it inside the house.

Leave any protective film on the glass until you’ve finished installing the door. [9] X Research source

Check the installation instructions that came with the door to make sure you have it facing the right way. They’ll tell you if the door is meant to open from right to left or from left to right.

The caulk will help stabilize the door, but one of you still needs to support it until it’s fully secured.

Use roofing or finishing nails to secure the shims. Countersink the nail heads so they don’t stick out beyond the shim.

The screws will typically be included with the door. If they’re not, the installation instructions for your door will tell you exactly what size nails or screws you need.

Check the door when you’re done to make sure it still opens and closes properly. If it doesn’t, the foam has moved it. Check your plumb and level again, then cut away the foam and start over. Use shims to readjust the door as necessary if it’s moved, then reapply your insulation.

If the shims are sticking out from the door on the inside, cut them down with your utility knife so the trim will fit properly.

If you adjusted the door up or down, you might have to move the latch and locks so they fit as well.

For most sliding glass doors, you have to buy a sliding screen separately. They’ll have a separate installation guide you can read through to make sure you’re installing it properly.